Excitation band-gap tuning of dopant based quantum dots with core-inner shell outer shell

ABSTRACT

A composition and method for fabricating and tuning a dopant based core-shell semiconductor having a quantum dot core with an excitation band-gap are provided. A quantum dot core composed of an alloy of cadmium sulfide (CdS) and zinc sulfide (ZnS) as semi-conductor materials include a dopant of manganese (Mn) added to the core and an outer shell of zinc sulfide (ZnS). The dopant based core/shell quantum dot semiconductor of the present invention allows the fine tuning of an excitation band-gap, covering a wide range (from 2.4 eV to ˜4 eV). When doped with Mn, these alloy Qdots emit bright yellow/orange light. Tuning of the excitation band is accomplished by changing the alloy composition of the core. Based on photophysical studies a new core/shell/shell model is provided, in place of the traditional core/shell model. Due to the interfacial diffusion of the cations from the core and shell an intermediate alloy layer is formed providing an inner shell; this inner shell layer is the real host of the dopant ions.

This is a divisional application of U.S. patent application Ser. No.12/275,269, filed Nov. 21, 2008 now U.S. Pat. No. 7,687,800 and claimspriority based on U.S. Provisional Application Ser. No. 61/004,138 filedon Nov. 23, 2007, the contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

This invention relates to fluorescent (or luminescent) quantum dots inthe field of nano-technology, and in particular to, tunable dopant basedcore/shell/shell quantum dots having a broad excitation spectrum,compositions and methods of fabricating. The research herein wassupported in part by the National Science Foundation (NSFCBET-63016011and NSF-NIRT Grant EEC-056560).

BACKGROUND AND PRIOR ART

Core/shell semiconductor quantum dots (Qdots) have attracted enormousresearch interest in the field of electronics, optoelectronics andbioimaging. To further diversify core/shell Qdot applications,especially in the field of spintronics, dopant based Qdots are ofparticular interest. By doping traditional CdS or ZnS core withtransition metal ions such as manganese (Mn), cobalt (Co), nickel (Ni)and the like, it is possible to develop dilute magnetic semiconductor(DMS) Qdots for spintronic applications as reported in recent scientificarticles written by D. A. Allwood et al in Science 2005 Vol. 309, pages1688-1692 and P. I. Archer in Nano Letters 2007 Vol 7, pages 1037-1043,for example.

It is expected that these dopant based binary DMS Qdots will advance thecapability of the next generation memory storage devices and computers.In this context, it is important to synthesize dopant based ternary DMSQdots, such as manganese (Mn) dopant based CdxZn₁-xS:Mn/ZnS Qdotheterostructures.

Fluorescent quantum dots (Qdots) have demonstrated their potential indiagnostic bioimaging applications in vitro as discussed by H. Yang etal. in Advanced Materials 2006, Vol. 18, page 2890. For in vivobioimaging applications, however, the embodiment of additionalproperties such as paramagnetism into the same fluorescent probe ishighly desirable. These multimodal probes would benefit in vivo diseasediagnosis and surgical guidance based on their ability to be detected inmultiple modes, such as, optically and magnetically. Thus, synthesis ofbright multimodal Qdots is a matter of great interest to a broad area ofthe scientific community from physics to bioscience.

The wide band gaps of the II-VI group semiconductors such as CdS and ZnSserve as good host materials for various kinds of foreign elements knownas dopants. Out of the different transition metals, manganese usuallyoccupies cadmium (Cd) or zinc (Zn) substitutional sites in the hostlattice as a divalent ion. The excitation and decay of manganese ionproduces a yellow/orange luminescence at approximately 590 nmwavelength, as reported by R. N. Bhargava et al in Phys. Rev. Lett.1994, Vol. 72, page 416 and S. Biswas et al. in Journal of PhysicalChemistry B, 2005, Vol. 109, 17256. This emission peak is generallyassociated with a transition between ⁴T₁ and ⁶A₁ energy levels. Also,the presence of the Mn²⁺ ions within the host Qdots introduces theparamagnetic property.

The realization that many molecular phenomena result in mechanicalresponses at the nanoscale level promises to bring about a revolution inthe field of chemical, physical, and biological applications. In a questfor smaller, faster, better, more accurate measuring and analyticaldevices there has been a wide application of traditional dopant basedQdots, particularly in biomedical imaging. However, the applications arelimited because of a relatively narrow excitation range, typically inthe UV range between 200-375 nm wavelengths.

In spite of the many advantages of dopant based quantum dotsemiconductors disclosed in scientific applications today, there arelimitations and disadvantages of the existing quantum dots regardingtheir adaptability and reconfigurablility. For example, traditionaldopant based Qdots in biomedical imaging with a relatively narrowexcitation range between 200-375 nm wavelengths is extremely harmful forbiological systems as excitation in this wavelength range can easilydestroy live cells. Again, due to narrow excitation ranges, these Qdotswill not be efficient for capturing a broad spectrum of solar light.Ideally, Qdots with broad excitation bands will eliminate theabove-mentioned limitations of traditional dopant based Qdots.

The performances of the QDots are often influenced by their surfacessince an appreciable portion of the constituent atoms reside at theirsurfaces for example, for a QDot with a diameter of 4 nm, 30% of itsatoms reside on the surface and thus are missing one or more of theirfour (tetrahedral) bonds to neighboring atoms. Chemically passivatingthese surface atoms and providing them with a true tetrahedral bondingenvironment plays a significant role in determining the optical andelectronic properties of the QDot.

A significant improvement in the performances of the QDots was realizedby growing a semiconductor shell around the core compared to the organicsurface capping ligands traditionally used to chemically passivate theQDot surface as discussed by H. Yang et al. in Advanced FunctionalMaterials 2004, 14, 152 and H. Yang et al. in Journal of ChemicalPhysics 2004, 121, 10233. Of the various types of QDots, Mn-doped II-VIQDots are of special attraction owing to their bright luminescence atroom temperature in the visible region. Also, presence of the Mn ion asa transition metal ion in the semiconductor host make them dilutemagnetic semiconductor (DMS), an interesting materials for applicationin the field of spintronics. Especially, the Mn doped type I core-shellsemiconductors are suitable for bio-imaging applications due to thelarge Stoke's shift in the emission spectra as discussed by S. Santra etal. in Chemical Communications 2005, 3144; S. Santra et al. in Journalof the American Chemical Society 2005, 127, 1656, and H. S. Yang et al.in Advanced Materials 2006, 18, 2890.

Performances of these doped semiconductors both as fluorescent as wellas spintronics materials depends on the position and distribution of theMn atoms inside the host lattice. Doping in the semiconductornanocrystals are often encountered with various difficulties due tovarious reasons including various kinetic factors, preferentialadsorption through specific surfaces etc. Difference in the ionic radiiof the substituent dopant and the substituted cation often introducessignificant amount of strain in the nanocrystal lattice; since strainfields are necessarily long range, much longer than typical nanocrystaldimensions, it tends to relieve itself by ejecting the dopant to thesurface of the nanocrystals. Thus, it is extremely important toinvestigate the positions of the Mn atoms inside a traditionalCdS:Mn/ZnS core/shell QDot.

The dopant based core-shell semiconductor quantum dots (Qdots) of thepresent invention solve many problems and overcome many limitations inthe prior art through alloying between the CdS and ZnS phase andproviding a different atmosphere to the substituent dopant.

SUMMARY OF THE INVENTION

The first objective of the present invention is to provide a dopantbased core/shell/shell semiconductor quantum dot (Qdot) with a broadspectrum excitation range.

The second objective of the present invention is to provide a tunabledopant based core/shell/shell semiconductor quantum dot (Qdot) with abroad spectrum excitation range.

The third objective of the present invention is to provide a dopantbased core/shell/shell semiconductor quantum dot (Qdot) efficient forcapturing a broad spectrum of solar light.

The fourth objective of the present invention is to provide a dopantbased core/shell/shell semiconductor quantum dot (Qdot) having a Qdotcore with an alloy of two semiconductor materials selected to possessvery similar crystal structure with minimal lattice mismatch andluminescence excitation spectra that partially overlap.

The fifth objective of this invention is to provide a dopant basedcore/shell/shell semiconductor quantum dot (Qdot) wherein the excitationband is tuned by changing the alloy composition of the Qdot core.

The sixth objective of this invention is to provide a dopant basedcore/shell/shell semiconductor quantum dot (Qdot) useful as ahigh-performance solar cell absorber material.

The seventh objective of the invention is to provide a dopant basedcore/shell/shell semiconductor quantum dot (Qdot) useful in sensitiveultra-small bioimaging probes.

The eighth objective of the invention is to provide a dopant basedcore/shell/shell semiconductor quantum dot (Qdot) useful as efficientelectroluminescent (phosphor) material.

The ninth objective of the invention is to provide a dopant basedcore/shell/shell semiconductor quantum dot (Qdot) useful in Spin-Basedelectronics (spintronics).

A tenth objective of the invention is to redesign the QDot structureswherein manganese (Mn) dopant atoms reside in an intermediate alloylayer of Cd_(x)Zn_((1-x))S formed in-situ at the interface of the coreand shell of QDots.

An eleventh objective of the invention is to provide aCdS/Cd_(x)Zn_(1-x)S:Mn/ZnS core/shell/shell model in place of thetraditional CdS:Mn/ZnS core/shell QDots.

A preferred dopant based core/shell/shell semiconductor having a quantumdot core with a tunable excitation band-gap includes a concentricarrangement of an alloy of cadmium sulfide (CdS) and zinc sulfide (ZnS)semiconductor materials in a core, a concentric layer composed in situformed Cd_(x)Zn_((1-x))S alloy embedded with the dopant manganese (Mn)forming an inner shell or interfacial layer surrounding the core, and anouter shell of zinc sulfide (ZnS).

The more preferred dopant based core/shell/shell quantum dot coreincludes an alloy of CdS and ZnS that varies from an approximately 100%ZnS core to an approximately 100% CdS core, when the alloy isapproximately 100% of ZnS and approximately 100% CdS, there isinterfacial diffusion of Cd and Zn atoms forming an additionalinterfacial alloy.

The more preferred dopant based core/shell/shell quantum dot coreincludes an alloy of CdS and ZnS and the dopant of Mn having acomposition selected from at least one of ZnS:Mn,Zn_(0.75)Cd_(0.25)S:Mn, Zn_(0.50)Cd_(0.50)S:Mn, Zn_(0.25)Cd_(0.75)S:Mn,and CdS:Mn.

The dopant based core/shell/shell quantum dot semiconductor of thepresent invention has an inner shell dopant that is only manganese (Mn)and the outer shell that is only zinc sulfide (ZnS).

The quantum dot core of the present invention has a core/shell/shellparticle size that is between approximately 3.0 nanometers (nm) toapproximately 3.5 nanometers (nm) in diameter with a core approximately2.0 nanometers (nm) to approximately 2.7 nanometers (nm) in diameter.

Further, the dopant based core/shell/shell semiconductor with quantumdot core of the present invention has an excitation spectrum that is ina wavelength range from approximately 275 nm to approximately 490 nm andthe excitation spectrum is efficient for capturing a broad spectrum ofsolar light.

The tuning of the excitation band of the dopant based core/shell/shellsemiconductor of the present invention is accomplished by changing thealloy composition of the core; the core is an alloy and the compositionof the interfacial alloy layer varies depending on the percentage of Cdand Zn inside the core.

A preferred method for fabricating a dopant based core/shell/shellsemiconductor having a quantum dot core with a tunable excitationband-gap includes selecting an acetate precursor of semiconductormaterials having identical crystal structures with minimal latticemismatch, adding a bivalent transition metal ion as a dopant, mixing theacetate precursor of semiconductor material and bivalent transitionmetal ion dopant in a first water-in-oil emulsion at room temperature,mixing a sulfide ion source in a second water-in-oil emulsion at roomtemperature, combining the first water-in-oil emulsion with the acetateprecursor of semiconductor material and bivalent transition metal withthe second water-in-oil emulsion containing the sulfide ion source, atroom temperature to form a quantum dot (Qdot) alloy core, and growing anouter semiconductor material shell layer about the Qdot core to form aternary alloy core/shell/shell dopant based quantum dot semiconductor.

The preferred precursor materials are zinc acetate, cadmium acetate andmanganese acetate. The preferred sulfide ion source is sodium sulfide.The preferred bivalent transition metal ion dopant is manganese (Mn),cobalt, (Co), and nickel (Ni), more preferably, bivalent transitionmetal ion dopant is manganese (Mn).

The preferred method for fabrication quantum dots in the presentinvention provides QDots wherein the average size of the ternary alloycore/shell/shell dopant based quantum dot semiconductor is in a rangebetween 3 nm and 4 nm in diameter.

Further objects and advantages of this invention will be apparent fromthe following detailed description of a presently preferred embodimentthat is illustrated schematically in the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a structural model of a conventional core/shell CdS:Mn/ZnSQdot. (Prior Art)

FIG. 2 is a structural model of a CdS/Cd_(x)Zn_(1-x)S:Mn/ZnScore/shell/shell of the present invention.

FIG. 3 is the CdS/Cd_(x)Zn_(1-x)S:Mn/ZnS core/shell/shell model showingexcitation properties of the dopant based core-shell semiconductorquantum dot (Qdot) of the present invention.

FIG. 4 a is the photoluminescence (PL) spectra of CdS:Mn and CdS:Mn/ZnSQDots.

FIG. 4 b is a graph of both the photoluminescence (PL) spectra and thephotoluminescence excitation (PLE) spectra of the CdS:Mn/ZnS QDots.

FIG. 5 a shows the photoluminescence excitation (PLE) spectra of CdS:Mn,CdS:Mn/CdS and CdS:Mn/ZnS QDots.

FIG. 5 b shows the photoluminescence (PL) spectra of CdS:Mn, CdS:Mn/CdSand CdS:Mn/ZnS QDots.

FIG. 6 a shows both the photoluminescence (PL) and photoluminescenceexcitation (PLE) spectra of ZnS:Mn and ZnS:Mn/ZnS QDots.

FIG. 6 b shows the photoluminescence (PL) and photoluminescenceexcitation (PLE) spectra of CdS:Mn and CdS:Mn/CdS QDots.

FIG. 6 c shows the photoluminescence excitation (PLE) spectra of ZnS:Mn,ZnS:Mn/ZnS, CdS:Mn and CdS:Mn/CdS QDots.

FIG. 7 a shows the PLE spectra of Cd_(x)Zn_(1-x)S:Mn QDots.

FIG. 7 b shows the PL spectra of Cd_(x)Zn_(1-x)S:Mn QDots.

FIG. 7 c shows the PLE spectra of Cd_(x)Zn_(1-x)S:Mn/ZnS QDots.

FIG. 7 d shows the PL spectra of Cd_(x)Zn_(1-x)S:Mn/Zns QDots.

FIG. 8 is the X-ray diffraction pattern showing direct evidence of corealloying. The legend shows the Qdot alloy composition.

FIG. 9 shows a representative high resolution transmission electronmicroscopic (HRTEM) image of one representative core/shell/shell Qdot.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before explaining the disclosed embodiment of the present invention indetail it is to be understood that the invention is not limited in itsapplication to the details of the particular arrangement shown since theinvention is capable of other embodiments. Also, the terminology usedherein is for the purpose of description and not of limitation.

Abbreviations and selected terms used herein are explained below:

Core/shell/shell is used herein to mean a concentric arrangement of thequantum dot structure, wherein there is a central core surrounded by aninner shell or interfacial layer that is then surrounded by an outershell.

CdS is the chemical formula for cadmium sulfide.

Mn is the chemical symbol for manganese.

QDot is an abbreviation understood in the industry for quantum dot.

ZnS is the chemical formula for zinc sulfide.

The present invention is a method for fabricating and tuning a dopantbased core/shell/shell semiconductor having a quantum dot (Qdot) corewith a tunable excitation band-gap. The inner shell is formed in situ bythe interfacial diffusion of the cations from the core and shell parts.

The present invention pertains to a novel design of the Qdot core.Unlike the traditional dopant-based, as well as, non-dopant basedcore/shell Qdots, the present Qdot core is atomically engineered toobtain a broad excitation spectrum. This is achieved by making the Qdotcore an alloy of two semiconductor materials. These two semiconductormaterials were selected in such a way that they possess very similarcrystal structure with minimal lattice mismatch and their luminescenceexcitation spectra partially overlap. Thus, by changing the alloycomposition of the Qdot core, the excitation band of the Qdot can betuned, providing a tunable dopant based core/shell semiconductor Qdot.

FIG. 1 is a structural model of a conventional core/shell CdS:Mn/ZnSQdot. The core consists of cadmium sulfide (CdS) with manganese (Mn)dopant and an outer shell of zinc sulfide (ZnS). In FIG. 2, the newnovel core/shell/shell model replacing the traditional core/shell modelof the QDot structure proposed in the present invention is shown. In thepresent invention the alloy composition of the core is varied from 100%ZnS core to 100% CdS core and can be represented by the general alloycomposition: Cd_(x)Zn_((1-x))S, and used in the detailed discussionbelow. The composition of the inner shell, formed in situ by theinterfacial diffusion of the cations from the core and shell partsdepends on the composition of the core as shown in FIG. 2. The preferredsite for the dopant Mn ions is the interfacial alloy CdZnS layer. Thisalloy layer is radially positioned in between the core and 100% zincsulfide (ZnS) layer.

The manganese (Mn) dopant in the core/shell/shell structure for Qdotsprovides bright luminescence.

For fabricating core/shell/shell Qdot hetero-nanostructure, the II-VIgroup semiconductors, such as cadmium sulfide (CdS) and zinc sulfide(ZnS) are attractive. Both the CdS and ZnS semiconductors have identicalcrystal structures with minimal lattice mismatch. Moreover, band-gapenergy of ZnS (3.6 eV for bulk) is higher than CdS (band gap 2.4 eV forthe bulk), making ZnS appropriate as shell material for synthesizingcore-shell CdS/ZnS nanostructures.

Cadmium sulfide (CdS) is one of the most studied nanostructures for itswide range of applications in electronics (such as nano-laser, thin filmtransistor, etc.) and optoelectronics waveguide, photonic circuitelement, photovoltaic materials, solar cells and the like, as reportedin scientific literature. Furthermore, for spintronic applications, anew type of DMS materials could be developed by doping/co-doping of CdSwith bivalent transition metal ions, such as manganese (Mn), cobalt(Co), nickel (Ni) and the like.

To demonstrate the engineering aspect of the above-mentionedcore/shell/shell hetero-nanostructures, in the present invention, a newtype of Mn dopant based core/shell Cd_(x)Zn_((1-x))S:Mn/ZnS Qdot isfabricated wherein the core is engineered to form an alloy. The Qdotcore engineering involved the modification of the binary semiconductorcore composition, such as ZnS:Mn or CdS:Mn, to form a ternary alloysemiconductor core having the composition: Cd_(x)Zn_((1-x))S:Mn.

A similar Cd_(x)Zn_((1-x))S ternary alloy structure is reported in theliterature and is related to thin film compound semiconductortechnology; Hsu et al. in Advanced Functional Materials 2005, Vol. 15,1350-1357; Z. Khefacha in Journal of Crystal Growth 2004, Vol. 260,400-409; R. Ahrenkiel, Solar Cells 1986, Vol. 16, 549-565.

However, the inventors herein are the first to fabricate dopant basedcore/shell/shell Qdots having a ternary alloy semiconductor core.

FIG. 3 is a schematic representation of emission properties ascribed toconventional QDot semiconductor materials when the core compositionconsists of an alloy of CdS and ZnS with a graphic representation ofexpanded photoexcitation and luminescence pathways made possible by theternary alloy semiconductor core of the present invention, having thecomposition Cd_(x)Zn_((1-x))S:Mn.

Upon excitation of the dopant based core-shell Qdots with externalphotons, the energetic electron could be promoted to the conduction bandof the CdS core, or Cd_(x)Zn_((1-x)) layer or outer ZnS layer. Electronsfrom all these conduction bands could populate the ⁴T₁ level of the Mn-dlevels via non-radiative decay. These excited electrons annihilate viaradiative transition between ⁴T₁=>⁶A₁ giving rise to brightyellow/orange emission at ˜590 nm.

With dopant Mn atoms residing in an intermediate alloy layerCd_(x)Zn_(1-x)S and formed in-situ at the interface of the core andshell of the QDots a Cd_(x)Zn_(1-x)S/Cd_(y)Zn_(1-y-)S:Mn/ZnScore/shell/shell model is prepared and analyzed for replacement oftraditional CdS:Mn/ZnS QDots.

The samples were characterized by room temperature photoluminescence(PL) and photoluminescence excitation (PLE) measurements. FIG. 4 adepicts the PL spectra of the Mn doped CdS quantum dots before and afterthe surface passivation through inorganic ZnS shell. As expected, asignificant increase of luminescence intensity was observed upon the ZnSshell layer deposition on the CdS:Mn QDot surface.

FIG. 4 b shows the room temperature PL and PLE spectra of the CdS:Mn/ZnSQDots exhibiting a strong emission at 592 nm corresponding to the Mn²⁺⁴T₁=>⁶A₁ d-d ligand-field transition. The PLE spectrum depicted in FIG.4 b is quite broad with half width at full maximum (FWHM) ˜140 nm.Similar broad PLE spectrum was also reported earlier for CdS:Mn/ZnSQDots prepared by following similar synthesis protocol as that of thiswork.^([14]) Authors have attributed the broadening of the peak to thesize distribution of the particles. But, the TEM image provided in thesame article showed the formation of quite moodispersed particlesindicating that the PLE spectral broadening is perhaps not only due tothe size distributions.

Another interesting observation made in FIG. 4 a is that theincorporation of the shell layer not only caused the PL enhancement butalso a clear peak position shift. With a view to investigate the effectof the shell layer a series of samples CdS:Mn, CdS:Mn/CdS and CdS:Mn/ZnSwere synthesized with the same W value (molar ratio of H₂O to AOT). TheW value was purposefully kept constant as it was supposed to be theresponsible factor for the particle size in reverse microemulsionsystem.

FIGS. 5 a and 5 b show the PLE and PL spectra of the CdS:Mn, CdS:Mn/CdSand CdS:Mn/ZnS QDots. The PLE spectra of the samples presented quite afew surprises which further disagree with the previous explanation thatthe PLE broadening is due to the size distribution of the QDots. The PLEspectrum of the CdS:Mn QDots presented a broad plot (FWHM ˜135 nm) withabsorption edge at ˜466 nm.

The PLE spectrum of the CdS:Mn/CdS QDots is quite sharp plot with FWHM˜42 nm. The PLE spectrum of the CdS:Mn/CdS QDots is little bit redshifted compared to the CdS:Mn QDots. This could be attributed to theincrease of the particle size due to the formation of the shell layer ofthe same CdS material as that of the core host material. But thesharpening of the PLE spectra of the CdS:Mn/CdS QDots is surprising ifwe consider the broad PLE spectra of the CdS:Mn QDots is due to the sizedistribution. This is because, if at all there exists a sizedistribution in the uncoated CdS:Mn QDots, the shell layer is veryunlikely to eliminate the size distribution. The shell layer is supposedto be of uniform thickness for all the individual particles and henceputting a shell layer on a poly dispersed particle system can notproduce a monodisperse core/shell particle system.

The second surprise came form the PLE spectrum of the CdS:Mn/ZnS QDotsin comparison with the uncoated CdS:Mn QDots. Although both the spectraare equally broad the spectrum for the CdS:Mn/ZnS QDots is quite a bit(˜35 nm) blue shifted compared to the uncoated samples. Since puttingthe shell layer always increase the particle size a blue shift which isin contrary to the expectation. Thus despite the increase of theparticle size, an increase in the energy gap for the CdS:Mn/ZnS QDotsindicated some changes in the electron transition route inside the core.This is because in type I semiconductor core/shell materials such asCdS/ZnS electronic transitions are supposed to be confined within thecore materials.

FIG. 5 b shows the peak normalized PL spectra of the CdS:Mn, CdS:Mn/CdSand CdS:Mn/ZnS QDots. All the spectra were peak normalized with a viewto compare their peak positions. The spectra shows that CdS:Mn andCdS:Mn/CdS QDots exhibited red light emission property with peakposition at ˜670 nm whereas the CdS:Mn/ZnS QDots exhibited ayellow/orange emission at ˜592 nm. This observation again indicated achange in the Mn environment i.e. in the core of the QDots upon ZnSshell layer coating.

In order to understand the changes in the PL and PLE spectra of theCdS:Mn/ZnS QDots, photoluminescence studies were also carried out onZnS:Mn, ZnS:Mn/ZnS, CdS:Mn and CdS:Mn/CdS QDots synthesized underidentical conditions. FIG. 6 a depicts the PL and PLE spectra of theZnS:Mn and ZnS:Mn/ZnS QDots. The PLE spectra for both the uncoated andcoated samples exhibited a sharp peak at ˜316 and 321 nm respectively.The red shift in the PLE spectrum of the coated sample is due to theincrease in the particle size. The PL of both the uncoated and coatedsample show a yellow/orange emission band at ˜592 nm corresponding tothe Mn²⁺ ⁴T₁=>⁶A₁ d-d ligand-field transition. The coated sample showsimproved PL intensity compared to the uncoated sample.

FIG. 6 b shows the PLE and PL spectra of the CdS:Mn and CdS:Mn/CdS QDotsrecorded with emission wavelength 670 nm and exciatation wavelength 468nm respectively. The PLE spectra of the CdS:Mn QDots are broad comparedto the CdS:Mn/CdS QDots as discussed earlier. Both the CdS:Mn andCdS:Mn/CdS QDots exhibited red emission at ˜670 nm and no peak positionshift was observed upon putting the same material (CdS) as the shelllayer over the host core (CdS) material. Again this red emission fromthe CdS:Mn and CdS:Mn/CdS QDots are also attributed to the same Mn²⁺⁴T₁=>⁶A₁ d-d ligand-field transition.

Red light emission for the CdS:Mn nanoparticles are reported by A. Naget al. in Chemistry of Materials 2007, 19, 3252. The red shift in the PLpeak position for the CdS:Mn compared to the ZnS:Mn QDots could beexplained in terms of crystal field splitting of the Mn d levels. Originof the Mn related emission peak is attributed to the spin fliptransition of 3d⁵ electron of Mn²⁺ ion from the threefold degeneratelowest level ⁴T₁(⁴G) (pulled down by the crystal field removingdegeneracy due to lack of symmetry) to the non-degenerate ground state⁶A₁(⁶S) level formed in the tetrahedral crystal field. The nature of thecrystal field in both ZnS and CdS is the same as in both Mn²⁺ aretetrahedrally coordinated.

In comparison with Mn doped CdS arrangement similar conditions arecreated as in the case of ZnS but with a stronger crystal field causinga red shift in the Mn²⁺ intra-level transition leading to red emission.A broadening in the PL spectra is observed due to the fluctuationscreated in the Mn²⁺ level by the crystal field.

The PL intensity increases for the CdS:Mn QDots after coating with theCdS shell layer but the enhancement was even better for the ZnS shelllayer. Thus although these observations failed to highlight the exactreason behind the PLE peak broadening but it is clear that it is not dueto size distribution. On the other hand the PL peak position shift forthe CdS:Mn/ZnS QDots compared to those of CdS:Mn and CdS:Mn/CdS QDotsclearly indicated a change in the Mn environment within the CdS:Mn/ZnSQDots. That means there could be two types of changes—either the Mnatoms are diffused to the ZnS shell layer or Zn atoms are diffused tothe core CdS layer to change the Mn atmosphere. The normalized PLEspectra of the CdS:Mn, CdS:Mn/CdS, ZnS:Mn/ZnS and CdS:Mn/ZnS QDotsexhibited in FIG. 6 c clearly shows that the Mn²⁺ ions within theCdS:Mn/ZnS QDots are neither in the pure ZnS environment nor in the CdSenvironment. To investigate the Mn environment in the CdS:Mn/ZnS QDots,a series of alloy semiconductor QDots in both uncoated(Cd_(x)Zn_(1-x)S:Mn) and coated form (Cd_(x)Zn_(1-x)S:Mn/ZnS) aresynthesized under the identical experimental conditions.

The PLE and PL spectra of these samples are depicted in FIGS. 7 a-7 d.The PLE spectra of the uncoated Cd_(x)Zn_(1-x)S:Mn QDOTS depicted inFIG. 7 a revealed the formation of a series of band gap tunable alloysemiconductor QDots through reverse microemulsion method. The spectrashow a gradual blue shift in the PLE spectra with changing compositionsfrom pure CdS to pure ZnS. The bluk band gap of CdS is 2.4 eV and thatof the ZnS is ˜3.7 eV and both of them possessed identical crystalstructure with Zn and Cd atoms in the tetrahedral coordination sites.

The ionic radii of Cd²⁺ and Zn²⁺ are very close to each other. Theseparameters make it easy to make a solid solution of CdS and ZnS phase invarious compositions and tune the band gap within the two extremelimits. In addition to the gradual red shift in the PLE spectra of theCd_(x)Zn_(1-x)S:Mn QDots with increasing x values another interestingobservation drawn from is the gradual broadening of the PLE peaks withthe increase in x value. Since all the samples were synthesized underthe same experimental conditions including the W values, it is veryunlikely that variation in the Cd/Zn ratio will introduce a systematicsize distribution. Thus, the broadening in the comparatively lower bandgap CdS QDots could be something else not the size distributions.

The PL spectra of the uncoated Cd_(x)Zn_(1-x)S:Mn QDots are depicted inFIG. 7 b. The figure shows that the PL peak position for the samplesremained almost unchanged at ˜592 nm for x value up to 0.5 but exhibitsa gradual red shift thereafter.

FIGS. 7 c and 7 d show the PLE and PL spectra of theCd_(x)Zn_(1-x)S:Mn/ZnS QDots. The PLE spectra of the core/shell QDotsshowed systematic redshifts with increasing x values but the PL peakposition remained unaltered. Thus, comparing all these PLE and PLspectra we can conclude that the Mn environment in the CdS:Mn/ZnSquantum dots are neither pure CdS nor pure ZnS but Cd_(x)Zn_(1-x)S withx value less than or equal to 0.5. Thus we propose that, in core/shellQDots, the core/shell boundary was not atomically distinct but anauto-generated intermediate alloy layer formed by diffusion of thecations from the core and shell layer.

FIG. 8 is the X-ray diffraction pattern showing direct evidence of corealloying. The gradual shift of the XRD peak indicated the core alloying.The legend shows the Qdot core composition. FIG. 9 shows arepresentative high resolution transmission electron microscopic (HRTEM)image of one representative core/shell/shell Qdot.

Nag et al. in J. Am. Chem. Soc 2008 (in press, DOI:10.1021/ja801249z)has recently reported that due to the differences in the ionic radii ofthe cations, MnS phase possessed lattice mismatch with the CdS and ZnSphase, which make it difficult to dope phase pure CdS or ZnS with higherconcentration of Mn.

The Cd_(0.51)Zn_(0.49)S phase possessed quite identical latticeparameter to that of the MnS phase and therefore Mn could be easilydoped in the Cd_(0.51)Zn_(0.49)S alloy nanocrystals. The latticemismatch induced lattice strain ejects the Mn atoms from the central CdScore of the QDots towards the surface in order to relax the latticestrain. The surface bound Mn atom then found the auto generated latticematched alloy phase as the host. Thus we propose aCdS/Cd_(x)Zn_(1-x)S:Mn/ZnS model in place of the traditional CdS:Mn/ZnSmodel. This new model further supports our experimental evidences as wehave observed a blue shift in the PLE spectra of the CdS:Mn/ZnS QDotscompared to that of the CdS:Mn QDots.

As Cd and Zn atoms on either side of the interface exchange theirposition through diffusion to form the intermediate Cd_(x)Zn_(1-x)Salloy layer, the effective core diameter gets reduced resulting in theblue shift in the PLE spectra.

In summary, we have experimentally established the formation of a alloyCd_(x)Zn_(1-x)S layer at the interface of the traditional CdS:Mn/ZnSQDots providing a new CdS/Cd_(x)Zn_(1-x)S:Mn/ZnS core/shell/shell modelfor the conventional core/shell model. In the process of establishingthe new concept of core/shell/shell structure we have synthesized aseries of dopant based alloy Cd_(x)Zn_(1-x)S:Mn andCd_(x)Zn_(1-x)S:Mn/ZnS quantum dots via the water-in-oil reversemicroemulsion methods. The core engineering led to a new type of Qdotswhere the Mn ions have a microenvironment of bivalent Cd, Zn and S ions,a tertiary alloy core, showing new photophysical characteristics.

Synthesis

The core/shell Cd_(x)Zn_((1-x))S:Mn/ZnS Qdots were synthesized using awater-in-oil (W/O) microemulsion method, consisting of dioctylsulfosuccinate, sodium salt (AOT, surfactant), water, and heptanefollowing a published protocol in the literature; H. Yang et al. inApplied Physics Letters 2003, Vol. 82, 1965-1967 and H. Yang et al inAdvanced Functional Materials 2004, Vol. 14, 152-156, incorporatedherein by reference.

To create Cd_(x)Zn_((1-x))S:Mn alloy core, a calculated amount of zinc,cadmium and manganese acetates were taken in one microemulsion andsulfide ion source in another microemulsion. Upon combining the twomicroemulsions a Qdot alloy core is formed.

The zinc sulfide shell layer was then grown by further addingappropriate amount of zinc source. We purposely took a surplus amount ofsulfur ion source to complete the core as well as the shell formation.

As a result of the above described alloying of Cd and Zn for the QDotcore, a series of excitation band-gap tunable core-shellCd_(x)Zn_((1-x))S:Mn/ZnS QDots are created. When x=0, we obtained pureZnS:Mn/ZnS core-shell QDots with highest excitation band-gap energy andsimilarly the lowest excitation band-gap energy was obtained with x=1.The photoluminescence excitation (PLE) of the dopant based core-shellQDots of the present invention is shown in FIG. 7 c andphotoluminescence (PL) spectra of the core-shell QDots of the presentinvention are shown in FIG. 7 d.

FIG. 7 c is the photoluminescence excitation spectra showing directevidence of excitation wavelength tuning of dopant based core-shellQDots. The inset shows the QDot core composition. An alloy corecomposition with Cd_(0.9)Zn_(0.1)S showed the broadest excitationspectrum in a wavelength range between approximately 275 nm andapproximately 490 nm. This excitation spectrum predicts that this willbe a great material for efficient capturing of solar energy for solarcell applications. Also, for bioimaging applications, this wideabsorption band will provide flexibility in selecting appropriateexcitation wavelengths. The PLE spectrum of ZnS:Mn/ZnS QDots was muchnarrower with a full width half maxima (FWHM) value 38 nm positioned at318 nm. With the increase of x value, we observed that the excitationband maximum shifted continuously towards the higher wavelength.Interestingly, we also observed an unusual broadening of excitation bandwith the increase of x value. The FWHM of the PLE spectrum for theCdS:Mn/ZnS QDots (i.e. x=1) was ˜123 nm. The change of the x value from0 to 1, however did not alter the ˜590 nm PL peak position, thusresulting in the formation of excitation band-gap tunable QDots.

The bright photoluminescence (PL) emission shown in FIG. 7 d isattributed to the Mn²⁺ ⁴T₁=>⁶A₁d-d ligand-field transition, suggestingrobust surface passivation of the QDot core by the epitaxially matchedZnS shell layer. The legend for FIG. 7 d shows the QDot corecomposition. Since all the phases of CdS, ZnS and Cd_(x)Zn_((1-x))S haveidentical crystal structures, the Mn²⁺ d-levels experienced similarcrystal field and therefore PL emission wavelength remained unchanged.Another way of stating the prior phenonenom, since the emission isoriginated from the Mn dopant excited state, as expected no emissionwavelength shift is observed. Since all the samples were excited at 355nm, as expected, we observed change in PL intensity with the x values.

Thus, unusual PLE peak broadening is due to alloying of QDot core. Sinceall the QDot materials were synthesized under identical microemulsionconditions, except the core composition, the crystallite size and sizedistribution will be very similar. Thus the large PLE peak broadeningfor higher x values could not be attributed to the QDot sizedistribution. The PLE spectrum of the ZnS:Mn/ZnS Q-dots was quite sharpand almost gaussian in nature. With the increase in the x values, thePLE spectra gradually broadened and deviates from the Gaussian shape. Athigher x values, we anticipate formation of at least two radiallydistributed crystalline microenvironments of hetero nanostructures, suchas the Cd_(x)Zn_((1-x))S alloy of the present invention with differentband gaps around Mn²⁺ ions, each contributing to the PLE spectrum. As aresult, we observed unusual PLE peak broadening.

Based on experimental findings herein, the formation of interfacialalloy layer of Cd_(x)Zn_((1-x))S is possibly justified in between CdScore and ZnS shell layer.

The QDot sizes were determined from the high resolution transmissionelectron microscopy (HRTEM) and average particle sizes were determinedas 3-4 nm in diameter for all core-shell compositions. The overallcore-shell particle size is less than 3.5 nm in diameter where the coreis about 2.7 nm in diameter.

The present invention is a bright yellow-emitting CdS:Mn/ZnS core-shellQDot with absorption maxima at 355 nm wavelength, as reported by H. Yangin Appl. Phys. Lett. 2003, Vol. 82, 1965 and S. Santra et al. in J. Am.Chem. Soc. 2005, Vol. 127, 1656. The reported excitation wavelength isin the UV range, limiting their wide application in cellular imaging.The present invention provides a core/shell/shell alloy semiconductorwith Cd_(x)Zn_((1-x))S inner shell and ZnS outer shell allowing the finetuning of excitation band-gap, covering a wide range (from 2.4 eV to ˜4eV). When doped with Mn, these alloy QDots emit bright yellow/orangelight.

A successful method and fabrication of broad excitation band-gap tunableCd_(x)Zn_((1-x))S:Mn/ZnS alloy semiconductor core/shell/shell QDotsusing a water-in-oil microemulsion technique is provided for the firsttime. Experimental evidence supports a core/shell (interface)/shellmodel instead of a traditional core-shell model.

The synthesis strategy can be extended to other semiconductor basedQDots to obtain new optical, magnetic and spintronic properties. Thequantum dots (QDots) of the present invention are appropriate for use indevices, apparatus, systems, and methods for measuring, analytical anddiagnostic fields, including, but not limited to, electronics,optoelectronics and bioimaging, spintronics, computers, and the like.

While the invention has been described, disclosed, illustrated and shownin various terms of certain embodiments or modifications which it haspresumed in practice, the scope of the invention is not intended to be,nor should it be deemed to be, limited thereby and such othermodifications or embodiments as may be suggested by the teachings hereinare particularly reserved especially as they fall within the breadth andscope of the claims here appended.

1. A method for fabricating a dopant based core/shell/shellsemiconductor having a quantum dot core with a tunable excitationband-gap consisting of the steps of: selecting an acetate precursor ofsemiconductor materials having identical crystal structures with minimallattice mismatch; adding a bivalent transition metal ion as a dopant;mixing the acetate precursor of semiconductor material and bivalenttransition metal ion dopant in a first water-in-oil emulsion at roomtemperature; mixing a sulfide ion source in a second water-in-oilemulsion at room temperature; combining the first water-in-oil emulsionwith the acetate precursor of semiconductor material and bivalenttransition metal with the second water-in-oil emulsion containing thesulfide ion source, at room temperature to form a quantum dot (Qdot)alloy core; and growing an outer semiconductor material shell layerabout the Qdot core to form a ternary alloy core/shell dopant basedquantum dot semiconductor said semiconductor comprising: an alloy ofcadmium sulfide (CdS) and zinc sulfide (ZnS) semiconductor materials toform a core; a concentric layer composed of in-situ formedCd_(x)Zn_((1-x))S alloy embedded with the dopant manganese (Mn) formingan inner shell surrounding the core; and an outer shell of zinc sulfide(ZnS).
 2. The method of claim 1, wherein the precursor materials areselected from at least one of zinc acetate, cadmium acetate andmanganese acetate.
 3. The method of claim 1, wherein the sulfide ionsource is selected from sodium sulfide.
 4. The method of claim 1,wherein the bivalent transition metal ion dopant is selected from atleast one of manganese (Mn), cobalt, (Co), and nickel (Ni).
 5. Themethod of claim 4, wherein the bivalent transition metal ion dopant ismanganese (Mn).
 6. The method of claim 1, wherein the average size ofthe ternary alloy core/shell dopant based quantum dot semiconductor isin a range between 3 nm and 4 nm in diameter.